As it is widely known, the power delivered by an internal-combustion engine depends on the amount of air fed to the combustion chamber of this engine, which amount of air is itself proportional to the density of this air.
Thus, it is usual to increase this amount of air through compression of the outside air before it is allowed into the combustion chamber. This operation, known as turbocharging, can be carried out using any means such as a turbocharger or a driven compressor, which can be a centrifugal or a positive-displacement compressor.
The turbocharger used for turbocharging comprises a rotary single-flow or double-flow turbine connected by a shaft to a rotary compressor. The exhaust gases from the engine flow through the turbine, which is then rotated. This rotation is thereafter transmitted to the compressor which, by its rotation, compresses the outside air before it is fed to the combustion chamber.
As is better described in French patent application No. 2,478,736, it is intended to increase the compression of the outside air by the compressor even further so as to be able to significantly amplify this amount of compressed air in the compression chamber of the engine.
This is achieved more particularly by increasing the rotational speed of the turbine and therefore of the compressor.
A fluid amplifier circuit, referred to as boost circuit, is therefore used, by means of which part of the compressed air exiting the compressor is diverted in order to be directly allowed to the turbine inlet while mixing with the exhaust gases. This turbine is then traversed by a larger amount of fluid (mixture of compressed air and exhaust gas), which allows the rotational speed of the turbine, and therefore of the compressor, to be increased. This compressor speed increase thus allows to raise the pressure of the outside air that is compressed in this compressor prior to being fed to the combustion chamber of the engine.
Thus, the compressed air has a higher density, which allows the amount of air contained in the combustion chamber to be increased.
This type of turbocharged engine, although satisfactory, however involves some not insignificant drawbacks.
Indeed, the flow of compressed air admitted at the turbine inlet is not correctly controlled, which may lead to dysfunctional engines.
Thus, by way of example, in case of too large amounts of compressed air diverted to the turbine inlet, the exhaust gases entering the turbine are cooled too much by this air, which causes a decrease in the overall turbocharging efficiency.
Furthermore, one of the major difficulties with the present turbocharging concept including a boost circuit lies in the compatibility thereof with exhaust gas recirculation. Indeed, most diesel engines are equipped with an exhaust gas recirculation circuit, referred to as EGR circuit, for limiting NOx emissions at source.
Exhaust gas recirculation is generally achieved by means of a HP EGR (High-Pressure Exhaust Gas Recirculation) circuit withdrawing the exhaust gas upstream from the turbine and sending it downstream from the intake air compressor. The recirculated exhaust gas circulating strictly in the opposite direction to the air diverted from the boost circuit, there is a likelihood of conflict between the two circuits, with the effects cancelling each other out. It is thus necessary to define a specific air loop architecture allowing the boost circuit and the HP EGR circuit to be made compatible.
Document EP-1,138,928 describes an EGR circuit and a boost circuit distinct in all respects, which requires complex construction and controls.
On the other hand, the present invention relates to an optimized air loop and exhaust gas recirculation architecture enabling to use, in a single engine, a HP EGR or a boost circuit, while avoiding too great a complexity for the respective lines and controls.